Not quite. It's not that the writers just accidentally forgot about them. They did write the OLTL characters out, but it was done in a hell of a hurry. They had to give the entire batch of GH characters collective amnesia that the OLTL characters had ever existed. The closest they got to acknowledging them was that one of them was the girlfriend of one of the long-time GH characters. They wrote her out by having her get a phone call from a friend in California, and she made an off-camera phone call to her boyfriend saying she had to leave and wasn't coming back. He received this call on-camera, and the girlfriend's name was never used. For a few weeks afterward, people just said, "Michael's girlfriend left town suddenly and isn't coming back, so that's why he's been in a rotten mood."
But now the actress is back and playing a different character and nobody mentions how much she resembles Michael's old girlfriend, since that character Never Existed.